Swelling in the lower legs, from the knee down to the ankles and feet, is almost always caused by fluid accumulating in the tissue. The medical term is peripheral edema, and it ranges from harmless (sitting too long on a flight) to serious (heart failure, blood clots, or kidney disease). The cause depends largely on whether one leg is swollen or both, how quickly the swelling appeared, and what other symptoms come with it.
One Leg vs. Both Legs
This is the single most useful clue. Swelling in just one leg typically points to a local problem: a blood clot (deep vein thrombosis), an infection like cellulitis, a sprained ankle, a Baker cyst behind the knee, or a blockage in the lymphatic drainage of that leg. Swelling in both legs usually signals something systemic, meaning a condition affecting the whole body. Heart failure, kidney disease, liver cirrhosis, and medication side effects all tend to cause fluid buildup in both legs equally.
There are exceptions. Chronic venous insufficiency, one of the most common causes overall, can affect one leg or both depending on which veins have damaged valves. Obesity-related swelling is almost always bilateral. Pregnancy causes swelling in both legs due to the added pressure on pelvic veins and increased blood volume.
Venous Insufficiency: The Most Common Culprit
The veins in your legs have one-way valves that push blood back up toward the heart against gravity. When those valves weaken or fail, blood flows backward (called reflux) and pools in the lower legs. This raises the pressure inside the veins, and that pressure gets transmitted into the tiniest blood vessels in your skin and tissue. Those small capillaries become more permeable, leaking fluid, proteins, and even red blood cells into the surrounding tissue. The result is persistent swelling, often worse by the end of the day and better after a night of sleep.
Over time, venous insufficiency also causes skin changes. You may notice a brownish discoloration around the ankles from leaked red blood cells breaking down, or the skin may become thickened and leathery. Varicose veins, visible and bulging near the surface, are a related sign that the venous system is under strain. This condition is progressive, so early management matters.
Heart Failure and Kidney Disease
When the heart can’t pump blood efficiently, the kidneys respond by retaining sodium and water in an attempt to increase blood volume and maintain circulation. This is a compensatory mechanism, but it backfires. The extra fluid expands both the bloodstream and the spaces between cells (the interstitial space), and gravity pulls that excess fluid into the lowest point: your lower legs and feet. Heart failure also reduces the concentration of albumin, a protein that normally keeps fluid inside blood vessels. With less albumin, fluid leaks more easily into surrounding tissue.
Kidney disease causes swelling through a similar but more direct route. Damaged kidneys lose their ability to filter waste and regulate sodium, so fluid simply accumulates. In nephrotic syndrome, the kidneys also leak large amounts of protein into the urine, which drops albumin levels and accelerates fluid leakage into tissue. Liver cirrhosis does something comparable by reducing the liver’s ability to produce albumin in the first place.
If your leg swelling comes with shortness of breath when lying down, waking up at night gasping for air, or rapid unexplained weight gain (several pounds over a few days), those are hallmarks of fluid overload from a heart or kidney problem.
Medications That Cause Leg Swelling
Several common medications cause lower leg swelling as a side effect. The most well-known are calcium channel blockers, a class of blood pressure medications. All drugs in this class can cause ankle swelling, with reported rates ranging from 1% to 15% at standard doses. At higher doses taken long-term, the incidence can exceed 80%. When a calcium channel blocker is combined with a certain type of blood pressure drug (an angiotensin system blocker), peripheral swelling drops by about 38% compared to taking the calcium channel blocker alone.
Other medications linked to lower leg edema include over-the-counter pain relievers like ibuprofen and naproxen, some diabetes medications, hormone therapy, and certain chemotherapy drugs. If swelling started within weeks of beginning a new medication, that’s worth flagging to whoever prescribed it. Stopping or switching the drug often resolves the problem.
Blood Clots: The Red Flag
Deep vein thrombosis (DVT) is swelling you should never ignore. A blood clot forms in a deep vein, usually in one leg, and partially or fully blocks blood flow. The classic signs are sudden swelling in one leg, pain or tenderness (especially when standing or walking), warmth in the swollen area, and skin that looks reddened or discolored. You may also notice that the surface veins look larger than normal.
The danger isn’t just the leg swelling itself. If the clot breaks loose and travels to the lungs, it becomes a pulmonary embolism, which can be life-threatening. Symptoms of that shift include chest pain, shortness of breath, coughing up blood, and feeling faint. Some people have no leg symptoms at all before the clot reaches the lungs. If you have sudden, one-sided leg swelling with pain and warmth, that warrants emergency evaluation, not a wait-and-see approach.
Prolonged Sitting and Standing
Even in a perfectly healthy person, gravity wins if you stay in one position long enough. Sitting for hours on a long flight or standing all day at work keeps blood and fluid pooled in the lower legs because the calf muscles aren’t actively pumping it back up. This type of swelling is temporary, symmetrical, and resolves once you move around or elevate your legs. It becomes a bigger concern for people who already have mild venous insufficiency or are on medications that promote fluid retention, since those factors compound.
How Pitting Edema Is Graded
When you press a finger into swollen skin and it leaves an indentation, that’s called pitting edema. Doctors grade it on a 1 to 4 scale based on how deep the pit is and how long it takes to bounce back:
- Grade 1: A 2 mm dent that rebounds immediately.
- Grade 2: A 3 to 4 mm dent that rebounds in under 15 seconds.
- Grade 3: A 5 to 6 mm dent that takes 15 to 60 seconds to rebound.
- Grade 4: An 8 mm dent that takes two to three minutes to fill back in.
You can check this yourself at home. Press firmly on the inside of your shin for about five seconds and release. Grade 1 or 2 after a long day on your feet is common and usually not alarming. Grade 3 or 4, especially if it’s persistent or getting worse, suggests a more significant underlying issue.
Reducing Swelling at Home
Leg elevation is the simplest intervention and it works for most causes of lower leg swelling. Position your legs above the level of your heart (propping them on a stack of pillows while lying on the couch works well) for about 15 minutes, three to four times a day. This uses gravity in your favor, helping fluid drain back toward the heart.
Compression stockings apply steady pressure to the outside of the legs, counteracting the tendency for fluid to leak out of blood vessels. They come in different pressure levels measured in mmHg. For most people with mild to moderate swelling, 20 to 30 mmHg is the standard starting range. Some people with more severe edema or lymphedema need 30 to 40 mmHg. Getting the right size matters as much as the right pressure, since stockings that are too tight at the top can actually worsen swelling below that point.
Sodium intake plays a direct role in how much fluid your body retains. For people with heart failure or kidney-related swelling, guidelines from the American Heart Association recommend keeping sodium under 2,000 mg per day when there’s active fluid overload. Even for people without a diagnosed condition, cutting back on processed foods and restaurant meals (the two biggest sodium sources in a typical diet) can noticeably reduce leg swelling within a few days.
Regular movement is equally important. Walking, even briefly, activates the calf muscles, which act as a pump to push blood upward through the veins. If you work at a desk, flexing your ankles up and down every 30 minutes mimics some of that pumping action. Avoiding long periods of crossed legs also helps, since that position compresses the veins behind the knees.